VXI VT1538A user manual About Period Frequency Measurements

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About Period and

Frequency

Measurements

The E1538A actually measures signal period for both the period and frequency functions. If the measurement function is set to frequency rather than period, the SCP returns the reciprocal of the measured period. The resolution of each period measurement is based on the time processor chip’s timer period (238.4nS). To improve resolution on faster input signals, multiple signal periods can be measured and averaged. For period measurements there are two different modes that can be used to control the number of periods to average. For frequency measurements only the APERture mode is available.

1.The [SENSe:]PERiod:NPERiod mode explicitly sets the number of signal periods to measure and average. The time it takes the SCP to return a reading is dependent on the input signal period (for a given NPERiod setting), longer signal periods take longer to return a reading.

In NPERiod mode the actual measurement resolution (in seconds) is fixed while the relative resolution (as a percentage of the input signal period) is variable. That is, when NPERiods is set to provide an adequate resolution for short period signals, long period signals will have increased resolution.

2.The APERture mode sets a fixed duration that the SCP will use to measure multiple signal periods. The actual effective APERture

<time> will be:

INT	⎛ --------<time>------------------------⎞	× signal_period .
	⎝ signal_period⎠

The minimum aperture will be 1 signal period, and the maximum will be 255 signal periods.

In APERture mode, the effective resolution (in seconds) varies with the period of the input signal. That is, as the signal period is reduced, the number of measurements averaged increases, thereby improving the effective resolution. However, the relative resolution (as a percentage of the input signal period) is fairly constant with changes in signal period.

Generally, more measurements (greater NPERiod count or longer APERture time) means a more accurate frequency value. Of course more measurements means that the reading returned contains more latency (is “older” in relation to the signal's current frequency). To track fast changing frequency, you have to trade-off some accuracy with a shorter aperture time.

16 Agilent E1538A Enhanced Frequency/Totalize/PWM SCP

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Contents VT1538A VXI Technology, Inc Introduction Support Resources User’s and Scpi Programming Manual Page Agilent E1538A Enhanced Frequency/Totalize/PWM SCP About this ManualIntroduction Identifying the Plug-on Important Setting Configuration Switches Switch Location and Example SettingsInstallation Connecting To The Terminal ModuleRecommended Signal Connections E1419A Option 11 Terminal Module ConnectionsInput and Output Characteristics Recommended Connections and Voltage LimitsVRS Mode Input Operation SCP Channels 0 & 1 only E1538A Input/Output CharacteristicsProgramming With Scpi Commands Shows the VRS mode input characteristicsConfiguring I/O Direction Setting the Input Threshold LevelDetermining the Input Threshold Level Inppol NormReading Static Digital State To set channels 40 through 43 to input digital statesTotalize Positive or Negative Edge State Changes Init SENSDATACVT? @44About Period Frequency Measurements SENSDATACVT? @45 SENSDATACVT? @45 Agilent E1538A Enhanced Frequency/Totalize/PWM SCP Agilent E1538A Enhanced Frequency/Totalize/PWM SCP Sense Quadrature Position Sensfuncquad 0,@148,149RPS Example of Rotational Velocity Sense Sense Rotational VelocityControlling Output Polarity Output Static Digital StateVariable Width Pulse Per Trigger Variable Width Pulse Train PWM Variable Frequency Fixed Width Pulse Train FM Variable Frequency Square-Wave Pulse Train Rotationally Positioned Pulse Output Rotational Pulse Command Usage Algupdate Init Example of variable position, preset width RST Sensfuncrvel 12,MISS,@140Variable Position, Width Preset by Time Example of variable width by angle, preset position Example of variable width by time, preset position Fixed Position, Variable Width by AngleALGSCALAR’ALG1’,’Width2’,.010 Stepper Motor Control Use the command Stepping mode valuesSensfuncvolt @100 Full Step Mode, Full and Half Speed, 4-Channel Page RST and *TST important Scpi Command Reference Command Syntax For newer Units onlyCommand Syntax For newer Units only Discussed INPutPOLarity INPutPOLarity?Parameters CommentsINPutTHResholdLEVel? Level can be set to a resolutionINPutTHResholdLEVel Send with VXIplug&play Function hpe14XXcmdReal64Q OUTPutPOLarityOUTPutPOLarity? SENSeFREQuencyAPERture SENSeFREQuencyLIMitLOWer Related Commands SensfreqaperSENSeFREQuencyAPERture? Init SENSeFUNCtionCONDition Related Commands INPutPOLaritySENSeFREQuencyLIMitLOWer? SENSeFUNCtionFREQuency SENSeFUNCtionPERiodSENSeFUNCtionPWIDth SENSeFUNCtionQUADratureSENSeFUNCtionRVELocity RPS SENSeFUNCtionTOTalize SENSePERiodAPERture SENSePERiodAPERture? SENSePERiodLIMitUPPerInit SENSePERiodLIMitUPPer? SENSePERiodMODESENSePERiodNPERiods Related Commands SENSePERiodMODESENSePERiodMODE? Send with VXIplug&play Function hpe14XXcmdInt32Q SENSePERiodNPERiods?SENSePERiodRANGeUPPer RST Condition is Send with VXIplug&play Function hpe14XXcmd SENSeTOTalizeRESetMODESENSePERiodRANGeUPPer? SENSeTOTalizeRESetMODE? SOURceFMSTATeSOURceFMSTATe? Send with VXIplug&play Function hpe14XXcmdInt16QSOURceFUNCtionRPULse SOURceFUNCtionSHAPeCONDition SOURceFUNCtionSHAPePULSeComments Send with VXIplug&play Function hpe14XXcmd SOURceFUNCtionSHAPeSQUareSOURceFUNCtionSTEPper Relationship of minvel, and maxvel SOURcePULMSTATe Usage See Stepper Motor Control onSOURcePULMSTATe? SOURcePULSePERiodSOURcePULSePERiod? SOURcePULSeWIDTh33,554,430 to 33,554,430 SOURcePULSeWIDTh?SOURceRPULsePOSitionANGLe Usage SOURceRPULsePOSitionANGLe?SOURRPULseVARType SOURRPULseVARType? SOURceRPULseWIDThANGLe00000787 to SOURceRPULseWIDThANGLe?SOURceRPULseWIDThTIME SOURceRPULseWIDThTIME? Specifications General SpecificationsFrequency Measurement Pulse Width Measure Rotational Pulse Source